Reference is here made to FIGS. 1 to 4 of the accompanying drawings. What these Figures represent is explained in the section "Brief Description of the Drawings" which appears further on in this document, and to which reference is invited at this point.
Referring to FIGS. 1 and 2, the linkage commonly comprises a cable 1 which is movable axially, a sheath 2 surrounding this cable, and a fork 4 which is pivotally mounted about a pivot or articulation point 7, which may for example consist of a knuckle piece secured to a fixed part of the vehicle as shown in FIG. 2, or a spindle which is rotatable and which is carried by a fixed part of the vehicle. In a variant, the articulation point may consist of bearings, with the fork then carrying projections which are rotatable in the said bearings, the latter being for example mounted in the gearbox casing.
Generally speaking the articulation point 7 constitutes a guide or datum point for the linkage.
The manoeuvring member is a clutch release bearing 3 which is subjected to the action of the declutching fork 4, and which is adapted to act on the declutching device of a clutch such as the ends of the fingers of a diaphragm 6 or declutching levers. The control member 8 most often consists of a pedal pivotally mounted about a pivot point 9, which is again fixed with respect to a fixed part of the vehicle.
The cable 1 is attached at each of its ends 10 and 11 to attachment points at which it is attached to the pedal 8 and the fork 4 respectively, while the sheath 2 is interposed between two points 12 and 13 that are fixed with respect to the vehicle.
The pivot points together with the fixed points will together be referred to in this document as "application points" for the linkage, which may include an hydraulic part, for example between the fork 4 and the clutch release bearing 3. These application points have the essential feature that they are ultimately carried by a fixed part of the vehicle, being secured to such fixed part or mounted rotatably with respect to it.
It is not obligatory to provide the fork 4, in which connection the point 11 may be attached to a disc in the manner described in French published patent application No. FR 1 587 732A; alternatively, it may be attached to a driving member in the manner described in U.S. Pat. No. 4,934,503 and the corresponding published European patent application No. EP 0 322 265A.
In FIG. 1, when the pedal 8 is depressed, this exerts a pulling force on the cable 1 which, guided by the sheath 2, actuates the fork 4.
Referring to FIG. 3, the point 11 may of course be fixed while the point 13 can be movable by virtue of being attached to the fork 4, so that when the position of the point 10 is varied, this involves a corresponding variation in the position of the point 13. In a modified embodiment, see FIG. 4, the points 10 and 11 may be fixed and the points 12 and 13 may be movable, being attached respectively to the pedal 8 and to the fork 4. The cable 1 then serves to guide the sheath 2 which is then axially movable. In all cases, one of the elements consisting of the cable and its sheath is a force transmitting element, while the other one of these two elements is a guide element.
In a further variant, the linkage can include a force transmitting element which is in several parts, having a plurality of bars with a return system and at least one articulation point carried by a fixed part of the vehicle, this articulation point then constituting a guide or datum point for the linkage.
In practice, the course of travel of the control member 8 and the manoeuvring member 3 can be broken down into a dead stage, a modulating stage with sliding movement, and a final stage to give complete coupling by the clutch. For example, and referring to FIG. 2, in the case of a conventional friction disc clutch 16, the dead stage occurs between the declutching position, in which the friction disc is free, and an intermediate position, referred to here as a take-up position, in which the friction liners 16' of the clutch disc 16 come into light contact with the pressure plate 15 of the clutch. The modulating stage occurs with circumferential sliding movement of the friction liners 16' between the reaction plate 17 and the pressure plate 15 of the clutch, and with an increasing amount of torque being transmitted until coupling is complete at a position referred to as the coupling position. The final stage enables good coupling to be achieved between the driving shaft M and the driven shaft B of the clutch. The driving shaft M is secured to the engine of the vehicle, for rotation therewith, and the driven shaft B is the input shaft of the gearbox. The modulating stage is thus valuable for passenger comfort and in facilitating manoeuvring of the vehicle, while the dead stage is necessary having regard to manufacturing tolerances of the various components of clutch, and also having regard to distortion effects, in particular the tendency of the pressure plate 15 to assume a slightly conical shape due to thermally induced forces to which it is subjected.
It should be mentioned here that the pressure plate 15 is rotatable with a cover plate 14, being movable axially with respect to the latter. This cover plate 14 is adapted to be carried on the reaction plate 17, while the diaphragm 6 bears on the cover plate 14 for action on the pressure plate 15 and for urging the latter towards the reaction plate 17, so that the friction pads 16' are gripped between the pressure plate 15 and the reaction plate 17. The plate 17 is secured to the engine or driving shaft M, while the clutch disc 16 is secured to, and rotatable with, the secondary or driven shaft B.
As the friction pads 16' become progressively worn, their thickness varies in such a way that the diaphragm 6 becomes inclined so as to displace the clutch release bearing 3. The latter then acts on the fork to exert an indirect pull on the cable 1. If no particular position is taken up, then with a constant linkage length, the result is seen particularly in the undue appearance of a clearance which causes uncontrolled slipping on the friction pads 16'. This is detrimental to the useful life of the clutch.
In order to overcome these drawbacks, it has already been proposed, in U.S. Pat. No. 4,304,322 and the corresponding French published patent application No. FR 2 420 164A, to divide the cable of the linkage into two successive pieces, the first of which is coupled with the declutching fork and the second piece being coupled to the control pedal. A compensating device is also provided, with the latter comprising two coupling members which are telescopically movable axially with respect to each other, and each of which is adapted to be coupled in movement to a respective one of the two parts of the cable in cooperation with disengageable locking means which are adapted to connect the said coupling members temporarily with each other, in only one axial direction of action of the latter.
Such an arrangement thus requires the cable to be cut in two and is itself subject to wear, because the disengageable locking means operates every time the driver operates the control member.